Is 400cc a double D size?

Madel,
If you are 4'10'' than maybe yes.  Cup size depends on your implant vs body size.  If you are very large frame then 400cc would definitely not be double D.
Daniel Barrett, MD, MHA, MSCertified, American Board of Plastic Surgery Member, Am. Society of Plastic Surgery

Hello and thank you for your question.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  This entire surgery
can be performed with a small incision technique.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.   The most important aspect is to find a
surgeon you are comfortable with. I recommend that you seek consultation with a
qualified board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon